RE: Windows NT does not check permissions after HANDLEs are open

[Michael Wojcik <Michael.Wojcik () merant com>]

> From: Syzop [mailto:syz () dds nl]
> Sent: Thursday, August 30, 2001 5:59 AM
> To: c0ncept () hushmail com
> Cc: VULN-DEV () securityfocus com; FOCUS-MS () securityfocus com
> Subject: Re: Windows NT does not check permissions after HANDLEs are
> open
>
> c0ncept () hushmail com wrote:
>
> >     The check against the ACL only occurs when the HANDLE 
> > is first opened, however. If a HANDLE is opened and 
> > permissions on the objecect subsiquently change, the original 
> > requestor of the object retains the original access-permissions.

> Isn't this normal?

Yes, it's pretty typical for discretionary access control (DAC) mechanisms
to perform access checking when a subject first requests access to an object
(eg. opens a file) and grant that access (if allowed by the DAC) for the
subject's lifetime or until it's specifically relinquished (eg. by closing
the handle).

In principle, a reference monitor should check every access, but "every
access" is pretty vague.  Should the monitor check every operation a subject
performs on an object - eg. every read and write on a file - against the
current ACL?  That would be expensive, so instead most systems establish a
boundary at which a subject (eg. a program, running with the rights of a
particular user, who may belong to one or more groups) is granted access to
an object for any number of subsequent operations (generally of certain
kinds - eg. you open a file for reading or writing or read-write).  Once the
subject is past that boundary the monitor remembers its decision for that
{subject, object, operation(s)} tuple and doesn't need to revisit the ACL.

This is indeed an issue for timely revocation of access, but it's a
well-known one, and arguably not much of a threat in most cases.  See for
example the aside about revocation in capability-based DAC implementations
in section 7.1 of the Rainbow book on DAC mechanisms.[1]


[1] http://security.isu.edu/isl/dac.html


Michael Wojcik
Principal Software Systems Developer, Micro Focus
Department of English, Miami University